(1) Field of the Invention
This invention relates to a production method of prefoamed particles of a polyolefin resin.
(2) Description of the Related Art
As a prefoaming method of particles of a polyolefin resin, it has been known to disperse particles of a polyolefin resin, which contains a volatile foaming agent, in water in a hermetic vessel, to heat the resultant dispersion under pressure to a temperature of at least the softening point of the particles, and then to release the resin particles and water into an atmosphere of a pressure lower than the internal pressure of the vessel while maintaining the internal pressure of the vessel constant with nitrogen gas (Japanese Patent Publication No. 1344/1981 published Jan. 13, 1981). Excellent prefoamed particles of the polyolefin resin can be obtained by the above method. However, the vacant space becomes greater in the vessel as the resin particle and water are released more and more from the vessel. As a result, the partial pressure of the volatile foaming agent in the vacant space in the vessel is lowered, so that the foaming agent is allowed to escape from the resin particles. Accordingly, the expansion ratio is lowered gradually as the prefoaming proceeds.
Incidentally, as an expansion-molding method making use of prefoamed particles of a polyolefin resin, it may be mentioned to subject the particle to molding either as they are or after an internal pressure is applied to the particles. When molding is conducted using particles having an increased internal pressure thus applied, expansion-molded articles can be obtained generally owing to the high expanding ability of the particles even if prefoamed particles of various expansion ratios are contained in combination. Even when such an internal pressure is applied in advance, there is an inconvenience that the expansion-molded articles have different physical properties. When molding is performed using particles which have not been subjected to the pre-treatment for the application of an internal pressure, the inclusion of prefoamed particles of varied expansion ratios leads to insufficient fusion bonding (primarily, on the side of lower expansion ratios) and/or shrinkage (mainly, on the side of higher expansion ratios) so that desired molded articles cannot be obtained. Whichever method is used, it is not preferable to conduct molding while prefoamed particles of various expansion ratios are included. It has hence been necessary to classify particles depending on their expansion ratios, especially, when molding is carried out using particles which have not been subjected to the pre-treatment for the application of an internal pressure. Such classification has however rendered the process very complex. Improvements have therefore been desired in this respect.
It has also been proposed to perform the release of resin particles while introducing a volatile foaming agent into a vessel, whereby the partial pressure of the volatile foaming agent in the space of the vessel may be kept constant and the expansion ratio of preformed particles, which are to be obtained, may also be maintained substantially constant (European Patent Application No. 95109 published Nov. 30, 1983). The volatile foaming agent is however required in a large amount in order to keep the partial pressure of the volatile foaming agent constant in the space of the vessel from the beginning of foaming until the end thereof.
Chlorofluorocarbon gases are used widely as foaming agents of the above kind for their non-combustibility in spite of their high prices. Reference may be had to the Examples of the European patent application referred to above. Use of chlorofluorocarbon gases in large volumes is however very questionable in view of production cost and ozonosphere destruction. According to this method, liquid dichlorodifluoromethane as cold as -30.degree. C. is introduced by way of example under pressure during prefoaming. As the water level becomes lower in a vessel during the prefoaming, more influence is given by the foaming agent of the low temperature. As a result, the water temperature drops and the foaming temperature can hardly be maintained constant. It is hence unavoidable that the expansion ratio decreases as the foaming of resin particles proceeds.